Securities Owned/Sold but Not Yet Purchased - Schedule of Securities Owned/Sold But Not Yet Purchased (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	$ 20,261	$ 17,319
Securities Sold, but not yet Purchased Total	469	582
Trading Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned Total	20,261	17,319
Securities Sold, but not yet Purchased Total	$ 469	$ 582